During storage, peanuts begin to become rancid and this rancidity becomes stronger with time. In addition, the peanut flavor fades away. The rancidity is known to occur from the oxidation reaction between peanut oil and oxygen. The reason for the flavor fading is not yet clear. One way of minimizing the oxidation reaction is to block oxygen entering the peanut through encapsulation technology, i.e. to coat the peanut with a certain wall material to reduce the rate of oxygen entering the peanut.
Many attempts have been made to coat nuts by spray coating technology to prevent the rancidity. The materials used in the spray coating are either high melting point fats (e.g. hydrogenated fats), or solvent (including water) soluble carbohydrates (e.g. starch, modified starches), or protein (e.g. whey protein, zein). The disadvantages of spray coating methods are (1) most of them are batch systems, (2) it takes a very skillful person to operate the systems to obtain a continuous and uniform coating on the surface of nuts. This is because spray coating is constructed by overlaying of droplets. Most of time, the overlaying droplets do not form a coating as continuous and uniform as it could be and, therefore, spray coating does not provide a complete barrier to oxygen.
Other attempts to coat nuts have used coating materials such as low melting point oils, e.g. soybean oil, cotton oil, coconut oil but in general these liquid oils do not have a sufficiently low oxygen permeability to protect the nuts from oxidation reactions to extend the shelf life.
We have found, surprisingly, that by inmersing nuts in a liquid edible coating material which solidifies at ambient temperature and then separating the excess coating material from the nuts, we can obtain a continuous coating on the surface of the nuts which not only surprisingly retards the formation of the rancidity but also, unexpectedly, retards the flavor fading.